Encyclia Cattleya hybrid leaves turning brown/black from top to base

[Shamu]

I’ve had this Cattleya peckhaviensis x Encyclia Paula Gross hybrid for about a year with no trouble. I saw new roots starting to emerge, so I repotted it in a mix with some sphagnum because it was drying out too quickly for me to keep up with the watering. Other than that nothing has changed in environment— it has a sunny south facing window with a humidity tray and watered with distilled water. However the leaves had been getting yellower, still green though, before repotting. Many of the roots were dry and brown (dead) when I repotted, so I cut them off with a sterile set of scissors (rubbing alcohol). I didn’t see any bugs and I still don’t see any.

Any ideas on what this might be? Should I be concerned?